1. Field of the Invention
This invention pertains to footwear in general, and in particular, to simplified, lightweight sole components for shoes and boots, and the methods by which they can be produced.
2. Description of the Related Art
Modern athletic and casual footwear typically incorporate some form of a resilient, composite sole component that is designed to achieve a balance, or compromise, between performance, comfort, looks and wear. Such sole components often include a foamed elastomeric midsole, which is typically made of a relatively low-durometer, thermoformed ethylene vinyl acetate ("EVA") plastic, or an injection-molded, foamed polyurethane ("PU") plastic material, or less frequently, a solid or foamed rubber elastomer, e.g., Neoprene. A recent patent that describes a method for making such a midsole of a particular composition of EVA material is described in U.S. Pat. No. 5,177,824 to Y. Ou.
Midsoles made of foamed EVA, PU, or rubber exhibit very good resilience and shock absorption properties, particularly in view of their relatively low weight and manufacturing costs. Also, they can be molded to include very intricate aesthetic and functional details, and will accept coloring fairly well by means of dyes, pits, additives and the like. However, one of their shortcomings is a poor resistance to wear, and particularly, a poor resistance to abrasion. Another drawback relates to their potentially poor coefficient of friction with certain types of surfaces, particularly after heavy wear, and the resulting loss of traction.
For these reasons, modern casual and athletic sole components usually also incorporate an outsole component molded of a solid, relatively high-durometer, resilient elastomer, typically, natural rubber ("NR") or man-made rubbers, such as butadiene ("BR"), styrene butadiene ("SBR"), or isoprene ("IR"), or combinations of these. These are typically laminated, or bonded, to the underside of the midsole by means of a flexible adhesive, as described below, and result in superior wear resistance and traction properties for the footwear.
It is known in the art, in a process called "direct attachment," to form either a PU midsole component or a PU outsole component in a first mold set, then to place the molded component into a second mold set and inject or pour the complementary sole component directly onto the first-molded component, such that the two components are chemically joined to one other after final curing. The resulting sole component is known as a "unit bottom," and the attachment between the outsole and midsole portions is very strong, since the respective molecules of the two portions are actually cross-polymerized, or linked, to each other. However, the method is limited to midsole and outsole components that are each made of a chemically similar material, namely, PU, and it is necessary to implement the second injection or pour step fairly quickly after the first-molded component is formed and before it is fully "cured," or polymerized.
However, midsole and outsole components are usually dissimilar in their materials, as described above. It is therefore typical to mold them in separate mold sets, then assemble them together adhesively, sometimes together with other sole or upper components, in a manual bonding, or adhesive, process wherein the cleanliness, "roughness," registration, and adhesive coverage of the two components, together with a strict control over the process temperature, are critical to an effective joining of the various parts. In U.S. Pat. No. 5,131,173, W. Anderie describes a sole component in which a relatively hard "carrier," or shank element, is adhesively interposed between an outsole component and a resilient midsole component for enhanced foot support in the finished shoe.
While it is possible to coat a first-molded sole component with a compatible adhesive, then inject the complementary sole component onto the adhesive layer in a second mold such that two parts are adhesively joined upon final cure of the second-molded component, little economy or efficiency is gained thereby, since the same strict control over the adhesive part of the operation must be maintained to ensure a reliable attachment of the components to each other. Therefore, it would be desirable if the adhesive, or bonding, operation could be eliminated entirely, particularly in the case of sole components of dissimilar materials, in favor of an outsole-to-midsole attachment process that is simpler, less costly, yet just as reliable.
Another problem indirectly associated with footwear sole component bonding methods is their finished weight. It is known that the wear experienced on an outsole component during normal wear of the shoe is restricted to certain areas on the underside of the sole. Because outsole materials are typically denser, and thus, heavier, than typical midsole component materials, it is at least theoretically possible to eliminate an appreciable portion of the overall weight of a sole component by confining the areal extent of the outsole to those regions of high wear, in a design that incorporates one or more distinct, unconnected "wear plugs," or discrete outsole components, and that leaves the underside of the midsole component exposed in areas of little or no wear. However, this theoretical design is seldom observed in conventional sole component construction because it effectively substitutes a plurality of adhesive bonds for what was previously a single bond joint, thus compounding the problems described above. Additionally, the individual wear-plug-to-midsole bond area is considerably reduced, relative to that of a design that incorporates coextensive midsole and outsole component surface attachment areas, thus reducing the effective tear strength of the individual wear plug.
Accordingly, sole components today, whether made by "direct attachment" or conventional lamination techniques, typically incorporate a design in which the outsole component covers substantially the entire bottom surface of the midsole, and wherein distinct wear plugs are interconnected by means of a common base, or "web," which provides a single, large-area surface that is attached to the bottom surface of the midsole component. While this design results in some benefits in production costs, namely, a simplified, "single-component" to "single-component" attachment, the countervailing penalty is additional material costs, and particularly, added weight, as the webbing typically accounts for about 50% of an outsole component's volume and weight. Again, since this web structure is only present to address the attachment problems described above, it is desirable to provide a method for reliably attaching discrete wear plug components to a midsole component that avoids the above attachment problems, including the need for this web structure.
The present invention relates to methods for producing footwear sole components in which one or more outsole components, or wear plugs, are reliably attached to at least one midsole component without the attendant material, structural, cost, and weight limitations of the direct attachment, or adhesive attachment, methods described above. The method results in sole components that are lightweight, reliably attached to each other, and yet relatively simple and inexpensive to make.